fix wrong bar scaling when in notebook and using unit_scale

[szymonlopaciuk]

When using tqdm.notebook.tqdm with the unit_scale option, the progress bar is not drawn properly, as total is multiplied by unit_scale twice. This PR aims to fix the problem.

Minimum example of the issue:

from tqdm.notebook import tqdm
from time import sleep

for _ in tqdm(range(0, 100, 10), unit_scale=10):
    sleep(0.1)